The invention relates to vacuum brake boosters with a vacuum chamber and a working chamber, which is separated in a pressure-tight manner from the latter by a moveable wall, and a control valve, which comprises a housing coupled in a working manner to the moveable wall and a first valve seat, which is disposed in the housing and which, interacting with a first valve sealing member, can control the supply of at least atmospheric pressure to the working chamber in accordance with the displacement of an input member, which is coupled to the first valve seat, of the brake booster in order to obtain a pressure difference at the moveable wall. Vacuum brake boosters of this kind have been known for a long time and are used in a vast number of cases to boost the actuating forces of a hydraulic vehicle brake system and thereby maintain them at a low level which is acceptable to the driver of a vehicle.
So-called brake assistants are also known. This term is usually understood to mean a system which can make increased braking power available to a driver with substantially the same actuating force in an emergency braking situation. Systems of this kind were developed because tests revealed that, although when emergency braking most vehicle users depress the brake pedal quickly, they do not do so forcefully enough to achieve the maximum possible braking power. The stopping distance of the vehicle is therefore longer than necessary. Systems of this kind which are already in production use a brake booster which can be electromagnetically actuated together with a device which can determine the actuating speed of the brake pedal. If this device detects an actuating speed lying above a predetermined threshold value, it is assumed that an emergency braking situation exists and the brake booster is fully driven by means of the electromagnetic actuating device, i.e. it provides its maximum boost power. A brake booster with an electromagnetically actuated brake assistant of this kind is known from DE 44 05 092 C1.
However brake boosters with an electromagnetic actuation facility are too expensive for motor vehicles of the lower and middle price category. Solutions which achieve a brake assistant function at less expense are therefore required.
The object of the invention is to provide a vacuum brake booster of the above-mentioned type with a brake assistant function without having to resort to a control valve which can be electromagnetically actuated. Unintentional initiation of the brake assistant function is at the same time to be prevented as far as possible.
Valve member which is coupled to the input member in the actuating direction of the brake booster. According to the invention the displaceable valve member is always subjected to the pressure prevailing in the working chamber at its rear side, which faces the input member. If, however, the displaceable valve member is displaced relative to the control valve housing in the actuating direction by more than a prefixed distance, then the displaceable valve member is subjected over at least a part of its front side, which lies opposite the rear side, to the pressure prevailing in the vacuum chamber, and the pressure difference then acting on the displaceable valve member holds the valve member in the position which is reached until pressure equalisation between the front side and the rear side of the displaceable valve member takes place.
This means that, according to the invention, the pressure difference prevailing at the moveable wall at the time is used to exert a force acting on the displaceable valve member in the actuating direction in order to hold the first valve seat formed at the displaceable valve member completely open in certain situations in which the actuating force exerted by the driver is not sufficient for this, so that the brake booster builds up the greatest possible pressure difference between its vacuum chamber and its working chamber, i.e. provides its maximum boost power.
After exceeding the above-mentioned, prefixed relative displacement, the displaceable valve member of the vacuum brake booster according to the invention is therefore virtually xe2x80x9csuckedxe2x80x9d into a position in which the first valve seat is completely open. However this necessary relative displacement is only achieved when the actuating speed of the input member exceeds a defined value. In the vacuum brake booster according to the invention the brake assistant function is therefore activated solely by the skilful utilisation of pressure differences present within the brake booster. No magnet is required to initiate the brake assistant function.
In the vacuum brake booster according to the invention the brake assistant function is disengaged by means of a reduction in the pressure difference acting on the displaceable valve member. The reduction in this pressure difference is initiated by a return movement of the input member which exceeds a certain measure.
In preferred embodiments of the vacuum brake booster according to the invention the displaceable valve member is resiliently biased opposite to the actuating direction of the brake booster. This resilient biasing advantageously ensures that the displaceable valve member is coupled to the input member in the actuating direction of the brake booster and on the other hand enables the displaceable valve member to be uncoupled from the input member when the brake assistant function is activated. In embodiments of this kind the force acting on the displaceable valve member on account of the pressure difference must be greater than the opposing spring force acting on the valve member in order to initiate the brake assistant function. This requirement can easily be taken into account by appropriately dimensioning the surfaces of the valve member which are subjected to the pressure difference.
A second valve seat is preferably formed at the displaceable valve member in the vacuum brake booster according to the invention, which seat co-operates with a second valve sealing member which in turn co-operates with a third valve seat which establishes a connection between the working chamber and the vacuum chamber in the open state. After the valve member has exceeded the prefixed displacement relative to the control valve housing, the second valve seat is closed and the third valve seat open, so that the pressure prevailing in the vacuum chamber can now act on the displaceable valve member.
In particularly preferred embodiments the second valve seat is formed at the front side of the displaceable valve member. An annular cavity is defined between the second valve seat and the third valve seat, both of which are preferably annular, the end boundary of which cavity is formed on one side by the displaceable valve member. The third valve seat is in particular disposed concentrically with and radially outside of the second valve seat. The annular cavity is connected to the vacuum chamber when the second valve seat is closed and the third valve seat open, while it is connected to the working chamber when the second valve seat is open and the third valve seat closed. The surface of the displaceable valve member which is located radially between the second valve seat and the third valve seat can thus be subjected either to the pressure in the vacuum chamber or the pressure in the working chamber.
According to one embodiment of the vacuum brake booster according to the invention, in order to be able to reduce a pressure difference acting on the displaceable valve member, the latter comprises a duct which connects the front side of the valve member to its rear side. This duct, which is normally closed, may be opened by displacing the input member or a component coupled to the latter, this displacement taking place relative to the displaceable valve member opposite to the actuating direction of the brake booster and exceeding a prefixed measure. The prefixed measure ensures that the brake assistant function is not unintentionally disconnected too soon.
According to a preferred configuration of the brake booster according to the invention, the duct connecting the front side to the rear side of the displaceable valve member can be closed by means of a ring seal comprising two axially spaced, circulating sealing lips. The ring seal is accommodated in a component which is guided on or in the displaceable valve member and can be displaced relative to the valve member. The axial spacing of the two circulating sealing lips in this case substantially determines the extent of the relative displacement between the component and the valve member which is necessary to open the duct and initiate pressure equalisation.
In preferred embodiments of the brake booster according to the invention the second valve sealing member is resiliently biased opposite to the actuating direction of the brake booster and can be axially displaced against this spring biasing. The extent of the axial displaceability of the second valve sealing member in this case represents the switching threshold which must be overridden in order to initiate the brake assistant function. In embodiments of this kind the force resulting from the pressure difference at the displaceable valve member must be greater than the sum of the spring forces which act in the opposite direction and which bias the second valve sealing member or displaceable valve member, respectively, in order to initiate the brake assistant function.
The displaceable valve member is preferably substantially sleeve-shaped in order to achieve a space-saving construction. The first valve seat is in this case formed at the end of the valve member which faces the input member, while the second valve seat is located at the opposite end of the valve member. A sleeve-shaped, displaceable valve member of this kind can be integrated into conventional control valve constructions without noticeably affecting the diameter or overall length thereof.
The input member is preferably also resiliently biased opposite to the actuating direction in all embodiments of the vacuum brake booster according to the invention. When the brake is released this resilient biasing returns the input member to the initial position. This resilient biasing of the input member is used to advantage in constructional terms during its return movement to the initial position to move back the above-mentioned component in which the ring seal is accommodated relative to the displaceable valve member in order thus to open the duct provided in the valve member and disconnect the brake assistant function.